The present invention generally relates to filtration apparatus and, in a preferred embodiment thereof, more particularly relates to a specially configured expandable well screen assembly for use in a subterranean wellbore, and associated methods of fabricating the well screen.
It is useful in some circumstances to be able to convey generally tubular equipment into a subterranean wellbore to a predetermined location therein, and then outwardly expand the equipment in the wellbore. For example, a restriction in the wellbore may prevent the equipment in its expanded configuration from passing through that part of the wellbore, but the equipment may pass through the restriction in its retracted configuration. in one application of this principle, it is known to use expandable well screens in wellbores.
An example of the potential usefulness of expandable equipment in a wellbore is where the wellbore intersects a productive, relatively unconsolidated formation. It is desirable in many situations to be able to utilize a well screen to filter production fluid from the formation, while foregoing the expense of cementing casing in the wellbore and performing a gravel packing operation. Unfortunately, without any radial support the unconsolidated formation would likely collapse into the wellbore, causing additional expense and loss of revenue. Conventional nonexpandable well screens must necessarily be smaller than the wellbore in order to be conveyed therethrough, and so they are incapable of providing any radial support for an unconsolidated formation.
Previously proposed expandable well screens have associated therewith several problems, limitations and disadvantages. For example, they are typically not designed for contacting and providing radial support for a formation, and are thus unsuited for this purpose. Additionally, at least one previously proposed well screen assembly construction has a multi-layer configuration in which various tubular elements must be telescoped with one another and then intersecured. The relative structural complexity of this previously proposed expandable well screen assembly, and the necessity of using multiple steps to fabricate it, undesirably increases its fabrication cost. Moreover, since the assembly portion outwardly circumscribing a perforated base pipe portion of the well screen has several layers, the maximum permissible unexpanded diameter of the base pipe is undesirably reduced due to the necessity of limiting the outer diameter of the well screen assembly to a maximum value determined by limiting well dimensions. Due to this reduced unexpanded diameter of the base pipe, operational expansion thereof undesirably increases the expansion stresses thereon and reduces the maximum available expanded diameter thereof.
As can readily be seen from the foregoing, a need exists for an improved expandable well screen, and associated fabrication methods, that eliminate or at least substantially reduce the above-mentioned problems, limitations and disadvantages of previously proposed well screen constructions as generally described above. It is to this need that the present invention is directed.
In carrying out principles of the present invention, in accordance with a preferred embodiment thereof, a specially designed well screen is provided and is useable in a subterranean wellbore as a particulate filtering structure. While the well screen is representatively of an expandable construction, it may also be advantageously utilized in applications where it is not necessary or desirable to expand the well screen. Additionally, principles of the present invention may be used in filtration applications other than in the representatively illustrated downhole well screen application.
According to an aspect of the invention, the well screen includes a perforated tubular base pipe coaxially circumscribed by a specially designed thin-walled tubular filter structure anchored to the base pipe and defined by a perforated tubular outer protective shroud having a tubular filter media sheet secured directly to its inner side surface. The construction of the filter structure facilitates the radial expansion of the well screen, provides it with a greater central flow area for a given maximum outer well screen diameter, simplifies the fabrication of the well screen, and reduces the fabrication cost of the well screen.
Preferably, the filter structure is of a metal mesh material and has relatively coarse radially outer and inner filter material layers between which a relatively fine intermediate filter material layer is sandwiched. The perforated tubular outer shroud member has a sidewall opening area percentage which is representatively in the range of from about 10 percent to about 30 percent, and is preferably about 23 percent.
According to a fabricational aspect of the invention in a preferred embodiment thereof, the tubular outer shroud/filter subassembly is formed by providing a flat perforated plate and placing on a side thereof a stack of individual metal mesh sheets. A diffusion bonding process is preferably used to bond the individual sheets to one another, and bond the sheet stack to the facing side of the perforated plate. Preferably, a single bonding process is used, although a first bonding step could be used to bond the sheets together, and a subsequent bonding step used to secure the bonded sheet stack to the perforated plate.
After peripherally trimming the flat plate/sheet stack subassembly to desired assembly dimensions, the plate/sheet stack assembly is deformed to a tubular configuration that defines the filter media-lined tubular shroud structure. A seam weld is placed along abutting edge portions of the now tubular perforated plate to hold it, and the tubular filter media structure which lines it and is directly secured to its inner side surface, in their finished tubular configurations.
The finished outer tubular shroud/filter structure is then placed coaxially around the perforated base pipe and suitably anchored thereto, for example by welding the opposite ends of the shroud to the base pipe, to complete the fabrication of the well screen.